The invention disclosed and claimed herein pertains generally to a system for detecting an anomalous target in an environment by emitting a search field comprising a series of electromagnetic pulses. More particularly, the invention pertains to such a system which is adapted to detect an anomalous target in a subsurface environment, such as a natural body of water or a portion of the earth's crust. Even more particularly, the invention pertains to a system for detecting a target in a subsurface environment, wherein a low frequency field which is backscattered from the target, and which is generated by a number of successive pulses, is isolated or separated at a detector from the scattering effects of individual pulses.
Radar is a well known example of a system which employs a pulsed search field to detect an anomalous target in the atmosphere. Pulsed field emission is required in order to provide intervals during which a portion of the emitted field, which has been reflected or backscattered by the target, can be detected at the site of pulse transmission without interference from the emitted field.
If an electromagnetic field is projected into a subsurface environment, such as an ocean body or a portion of the earth's crust, the frequency thereof must be in the Extremely Low Frequency (ELF) range, on the order of 1 Hz-100 Hz, to prevent the field from being substantially attenuated. The projection of an electromagnetic field at such low frequency is generally quite difficult, due to the cumbersome nature of the antenna and related equipment required therefor. Also, the time period of a single sinewave of an ELF field is comparatively long, e.g., 1 sec for a frequency of 1 Hz. In a search system of such low frequency, it is not possible to isolate a field backscattered by a target from interfering effects of the primary or emitted field of the system, as is done in radar. The distance traveled by a single emitted sinewave is so great that a target anomaly detected by the system must be located a very great distance from the point of emission to enable the system to operate. For example, for a sinewave of 1 Hz frequency target distance must be on the order of half the distance of the moon from the earth.
To provide a pulsed field technique for mineral exploration in the earth's crust, which overcomes the above problems, a prior art system projects a primary field comprising high repetition rate into the earth. If conductive ore bodies are contained therein, each emitted pulse generates an eddy current in the ore bodies. Then, during the interval following a pulse, the eddy current decays, generating a detectable transient magnetic field. Such secondary transient field is sampled during the interval at a high sampling frequency, and the sampled data is processed according to conventional techniques to provide information locating the ore bodies. However, in such system, a very large part of the secondary field is discarded, since it is generated during the period of the primary pulse.
In other systems for geological applications, a complex primary field is projected into the earth, and a received secondary field is matched with waveforms representing various geological structures to determine the best fit therebetween. Such systems, however, may require very elaborate computer systems and special algorithms.
In the present invention, a pulsed field search system is provided which senses the presence of an anomalous target in a subsurface environment solely by the detection of a field of very low frequency which has been backscattered from the target. Data processing equipment required for the system is greatly simplified, and an embodiment thereof may be very usefully employed to detect metallic or other
anomalous targets in natural bodies of water, such as ocean bodies. It is anticipated that an embodiment of the invention may also be employed to detect tunnels or other voids in a portion of the earth's crust.